The present invention relates to a process for the preparation of Y--Ba--Cu--O superconductors, and more particularly to a process for the preparation of the precursor used in the fabrication of Y--Ba--Cu--O superconductor by sol-gel technology.
In January of 1987, Wu et.al. (Phys. Rev. Lett., 58 ((1987)) 908) and Hor et. al. (Phys. Rev. Lett., 58 ((1987)) 911) discovered that Y--Ba--Cu--O sintered body can show superconductivity while below 90 degrees Kelvin. Since then, the studies about the preparation of this superconductor have received great attention in the field of physics, chemistry and materials. Among them, the sol-gel technique is particularly interesting to many study groups, because in general, the sol-gel technique offers the unique feature of being able to control the stoichiometry and chemical homogenity.
Within the sol-gel technique, the alkoxide method is well known. Takashi et. al. (Chemistry Letters, pp. 287-290, 1988 ) fabricated a Y--Ba--Cu--O superconducting thin film using this method. In the fabrication, metal alkoxides consisting of yttrium butoxide, barium methoxide and copper methoxide in a given atomic ratio were dissolved in a triethanolamine-methanol solution. Coating of the resultant solution, heating at 800.degree. C. in air and annealing in an oxygen atmosphere resulted in a superconducting oxide thin film. However, for dissolving the metal alkoxides, pH should be controlled at an appropriate value, otherwise the formation of the product will be inhomogeneous. In addition, the alkoxides of yttrium, barium and copper are difficult to prepare because they easily to hydrolyze in the atmosphere. In Takashi's et. al. process, in order to prepare a metal alkoxide solution, the composition of which has a certain ratio, Ba metal was put in methanol under the reflux in N to make barium methoxide in methanol. Separately, a solution of Y(O--nC.sub.4 H.sub.9).sub.3 in xylene was added to the mixture of methanol and triethanolamine and the resultant solution was added to the barium methoxide-methanol solution. Then, Cu(OCH.sub.3).sub.2 was stirred into the resultant solution to obtain the dark blue starting solution. The process is too complicated to be industrilized.
Wang et. al. (Inorg. Chem., 26 ((1987)) 1474) prepared La--Sr--Cu--O superconductors using the nitrate method, respectively via the carbonate route, the citrate route and the oxalate route. In the carbonate route, powdered samples of La(NO.sub.3).sub.3.6H.sub.2 O, Sr(NO.sub.3).sub.2 and Cu(NO.sub.3).sub.2.3H.sub.2 O were dissolved together in distilled water. The nitrate solution was vigorously stirred and neutralized to pH 7-8 in order to suppress bicarbonate formation by titration with a KOH solution. A solution of K.sub.2 CO.sub.3 in water was slowly stirred into the above mixture, resulting in a thick, light blue precipitate. The precipitate was repeatedly washed and centrifuged until the filtrate had a pH of less than 8.8. The resulting blue powder was dried at 140.degree. C. overnight and calcined at 820.degree. C. for 2 hours. The black powder was ground and pressed into pellets and sintered at temperatures up to 1100 C. in air for 4 hours. Sections of the pellets were then annealed at 500.degree. C. in an Argon/oxygen gas mixture.
In the citrate route, the same powdered metal nitrates were dissolved in distilled water. Then, powdered citric acid and ethylene glycol were added to the nitrate solution. The resulting blue solution was stirred and heated to 90.degree. C., at which temperature a vigorous reaction occured and copious amounts of NO.sub.x gas were evolved. The heat was turned off while the stirring was continued. The reaction was complete within about 2 hours, and the solution began to set into a gel. The gel was subsequently decomposed to a solid by heating it in a Pyrex beaker on a hot plate. The resulting black solid was ground into a powder finer than 20 mesh. This powder was pressed into pellets, sintered and annealed according to the previously described methods.
In the oxalate route, the procedure generally followed the steps employed in the carbonate method, except for the use of potassium oxalate for the precipitation.
Up to now, however, no process for the preparation of Y--Ba--Cu--O superconductors by the nitrate method has been reported.